The extensions system follows many of the rules from the main build system. For this particular example, helpers are defined in the components/common portion of the App Inventor code base, and these classes are shared between both the Android code and the client side code that runs in the browser. Therefore, it was never intended that the Android classes be available (since the web browser is not an Android environment). This will "fix" your issue at the expense of not being able to build the whole of App Inventor.
I understood! Thank you very much for your considerations.
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